Axial flux permanent-magnet synchronous machines (AFPMSM) have become popular in the last few decades because they may have a higher torque at low speed in comparison with radial flux machines with inner rotor. In very special designs, outer rotor designs may also compete in torque/weight or in efficiency. AFPMSM can for example be attractive when the axial length is small and the pole number is high. AFPMSMs make it possible to implement a high number of poles. Even at low rotation speed, the voltage per turn can be high enough to obtain a given power with a rather low stator current, and a low number of turns.
The property of high torque at low speed is interesting for several applications, such as traction and energy generation. The AFPMSM is investigated as a high efficiency generator for e.g. wind energy systems, in particular “direct-drive” generators with direct coupling to the shaft, causing the nominal speed of the machine to be very low. For example, a single rotor single stator as well as a single rotor dual stator AFPMSM have previously been designed for direct-drive wind applications. In literature, during the last years, many aspects have been investigated.
Cogging torque reduction and optimisation of the flux generated per winding are two examples of aspects which have been studied intensively in literature. In JP2005-348552 a number of sets of identical laminates are stacked to form a tooth of a stator having a T-shape. Within each set, the laminates are stacked in such a way that they completely overlap. The remaining holes between the different teeth still results in a cogging torque which is substantially high. In order to further reduce the cogging torque, the remaining holes may be filled with a magnetic powder. However, powder particles are likely to come loose during use because of interaction with the magnets of the rotor, resulting in damage or destruction of the electrical machine. Moreover, such particles typically show a lower permeability and higher losses compared to the solid materials. Hence, magnetic powder is only a partial solution.
A number of manufacturing techniques for stator teeth are known. Some are based on the use of laminates and stacking of such laminates in order to form appropriately shaped stator teeth. Stacking of such laminates nevertheless often is tedious and time consuming and there is room for improvement.
Although a number of approaches have already been described, there still is room for further optimisation of the axial flux synchronous machine.